The invention relates to a method and apparatus for monitoring run/stop conditions of a yarn in a knitting or warping machine.
In order to detect yarn breakage in textile machines, like knitting or warping machines, a yarn feeler is known which is able to output a logical final output signal indicating the run/stop conditions of a yarn actuating a transducer. A typical structure of a yarn feeler includes the transducer, a variable gain amplifier, a detector/comparator operating with a threshold in order to gain a detected run signal and an output filter operating with a predetermined time delay to output final output signals. The electrical run input signal of the transducer will mainly be generated on the basis of the yarn speed but also on the basis of other parameters like yarn tension, yarn linear specific mass, yarn count, yarn flexibility, yarn surface roughness, electrostatic charge of the yarn, etc. A variable gain amplifier is used because the amplification gain needs to be adjusted towards a minimum just assuring a stable output signal irrespective of parametric natural influences. A gain amplification which is too strong results in a poor time definition of the output and an output sensitive to spurious yarn motions simulated by external noise. A gain amplification which is too low results in an erratic output signal despite a correct run of the yarn. In the known yarn feeler the variable gain amplifier is adjusted manually. However, this is not well accepted by the users, because such empirical adjustment or trimming procedures are a waste of time and require particular skill, especially if a plurality of yarn feelers are installed at a machine. On the other hand, there is always a large risk that the adjustment is not carried out correctly.
It is an object of the invention to provide a method as disclosed and a yarn feeler which operates on the basis of this method, both leading to the highest quality of yarn monitoring, i.e. to avoid a poor time definition of the output signal, to achieve output signals insensitive to external noise, and to safely avoid an erroneously generated final output stop signal in case of a proper run of the yarn.
According to the method of the invention, the gain amplification permanently and automatically is adjusted to an optimum, namely a minimum just sufficient to ensure stable final output signals. No manual adjustments are necessary. Since the yarn feeler is adapting itself to an optimum sensitivity assuring stable final output signals, poor time definitions of the output signals and influences of external noises are avoided as well as an erroneously generated final output stop signal in case of properly running yarn. Said minimum is permanently adapted to instantaneously cope with all influencing parameters.
The yarn feeler does not need any manual trimming or adjustments since it automatically is seeking an optimum gain amplification. In knitting or warping machines having a plurality of such yarn feelers, the quality of each yarn feeler in view of its operation behaviour is enhanced significantly. The improved monitoring quality is achieved without the need for adjustment procedures carried out by operators. Of particular advantage is that a change of the yarn count or the yarn quality does not need any preparatory work at the yarn feelers since each yarn feeler has its own self-learning control adapting automatically to the instantaneous conditions and influencing parameters. The control strategy used is an automatic gain control technique interfering in a regulating fashion at the variable gain amplifier in order to maintain the final output signal within specified limits and independently of the amplitudes of the run input signal. A prerequisite is that the control band width is larger than the band width of the input run signal variation such that the control is able to follow these natural parametric variations. The control is operating with a constant reaction time. In order to avoid false output stop signals during normal run of the yarn, the output signals are filtered with a time delay slightly longer than the reaction time of the control. Said additional delay is acceptable for applications where yarn speed variations are moderate and also where the top speed of the yarn during the run is predeterminably moderate as on knitting or warping machines. Any type of electronic transducer can be integrated into the yarn feeler like piezo-electronic, electrostatic or other transducers. A final prerequisite of a correct function is that the band width of signals caused by yarn breakages is by far larger than the control band width. A yarn breakage will lead to an input run signal drop occurring much faster than the reaction time of the control so that a correct final output stop signal will result safely.
Particularly in knitting or warping machines, the natural parametric variations are slow enough, since the yarn starts its run with a mild acceleration, runs for a long time at essentially constant speed, until it then stops after a smooth deceleration. The slowness of the physical phenomenon provides enough time to adjust the gain amplification without the danger of generating false final stop signals, namely by filtering with an acceptable time delay prior to putting out the final output signal.
It is advantageous to compare the amplified run input signal with a predetermined threshold in order to output a detected run signal, on the basis of which the final output signal can safely be generated, but which simultaneously can be used to control the gain amplification such that the amplified run input signal is just higher than the threshold. As already mentioned, the mutually related band widths of the control and the natural variations of the run input signal allow the control to follow such variations in order to reliably achieve an essentially stable detected run signal, fluctuations of which are filtered by the output filter as long as such a fluctuation is not caused by a fast breakage drop.
According to a further aspect of the method, the variations of the gain amplification are controlled independently from the amplitudes, of the run input signal in order to keep the final output signal within specified limits.
Said AGC-control strategy can be carried out reliably and permanently by generating an amplification gain control signal on the basis of the detected run signal, to which amplification gain control signal the amplifier is responding by varying its amplification factor or sensitivity accordingly. As soon as the detected run signal shows the tendency to rise or to fall, the gain amplification will be lowered or raised accordingly.
Since in the case of a piezo-electric transducer almost all parameters originating from the yarn and its run are essentially constant, except the yarn tension decisive for the run input signal, the amplification gain control signal generated on the basis of the detected run signal is reflecting relatively precisely the control effort necessary to compensate for tension variations. Said interrelationship can be used to measure the instantaneous yarn tension.
In order to generate a reliable, logical, detected run signal or run/stop signal it could also be necessary to vary the detection threshold.
Since a final output stop signal also can occur within the correct operation cycle of the machine equipped with the yarn feeler, namely when the yarn is stopped as intended but not due to a yarn breakage, it is useful to evaluate the final output signals representing the run/stop conditions of the yarn in view of a sync-signal associated with normal or correct run/stop conditions. A final output stop signal-representing a yarn breakage leads to a stop of the machine when the associated sync-signal is indicating that the yarn should run.
In the yarn feeler it is advantageous to have a reaction time of the AGC-control strategy weak enough to compensate for natural parametrical fluctuation or spikes in the detected run signal, which fluctuations, as mentioned, occur slowly enough. Since to the contrary, a yarn breakage leads to a sudden drop of the yarn input signal, the then detected run signal cannot be maintained stable further on, and even the output filter cannot filter out said sudden drop, such that in the case of a yarn breakage a reliable final output stop signal will be generated.
The reaction time of the amplification gain control circuit ought to be adapted to the compensation of natural parametrical fluctuations.
Any type of transducer can be used for the yarn feeler. Of particular advantage are piezo-electric or electrostatic transducers which operate reliably and safely.